Comparison of six-degree-of-freedom registration errors across different anatomical registration regions in head and neck radiotherapy under identical CBCT conditions.

objectiveCone-beam computed tomography (CBCT) has been widely adopted for patient setup evaluation and error correction in head and neck radiotherapy. Although different anatomical registration regions are routinely used in clinical practice, the characteristics of six-degree-of-freedom (6DoF) setup errors associated with different registration regions have not been systematically compared within the same CBCT acquisition. This study aimed to compare the distributional characteristics of 6DoF setup errors across different anatomical registration regions using a within-scan comparison framework and to describe their structural features from a multivariate perspective.

methodsCBCT data acquired during the radiotherapy course of patients with head and neck cancer were retrospectively analyzed. For each CBCT scan, multiple anatomical registration regions were independently applied based on the same image dataset, and the corresponding 6DoF setup errors were recorded. Paired statistical methods were used to compare the distributions of setup errors among different registration regions. Principal component analysis (PCA) was further performed to characterize the overall structural patterns of the 6DoF error components.

resultsWithin the same CBCT acquisition, the distributions of 6DoF setup errors differed among anatomical registration regions. The observed differences were mainly reflected in certain rotational components, whereas the differences among translational components were relatively less pronounced. PCA results demonstrated that the structure of 6DoF setup errors could be characterized by different variation patterns dominated by either rotational or translational components. Although partial overlap was observed among registration regions in the principal component space, differences in their distributional characteristics were still evident.

conclusionsWithin the same CBCT acquisition, the selection of anatomical registration regions may influence both the distribution and structural characteristics of 6DoF setup errors in head and neck radiotherapy. These findings suggest that the potential impact of registration region selection should be considered during clinical setup evaluation.